Time-to-digital converters (TDCs) are used to measure time intervals and convert them into digital output signals. A single-photon avalanche diode (SPAD, also known as Geiger-mode avalanche photodiode) is a photodetector in which a photon-generated carrier triggers an avalanche current.
CA 2 562 200 A1 discloses a time-to-digital converter comprising digital delay circuits. A clock compensation scheme is used to modify and adjust the operation of the TDC. A digital processing algorithm produces one conversion every clock cycle. The time-to-digital converter is intended for high speed circuit applications such as time-based analog-to-digital converters for conversion of radio frequency signals in wireless communication systems and high speed signal measurement applications.
WO 2016/035469 A1 discloses a time measurement device calculating the time between first and second trigger signals. It is provided with start and stop gates for generating a start signal and a stop signal, a TDC circuit for generating a digital code corresponding to the time between the input of the start signal and the input of the stop signal, a delay circuit for delaying the input of the start signal and/or the stop signal to the TDC circuit by a prescribed delay time, and a control unit for calculating a measurement time on the basis of a plurality of digital codes generated by the TDC circuit.
TDCs are typically reset according to fixed measurement periods of constant duration. Thus a TDC either measures the time interval from the beginning of one of the fixed periods, which is used as a start-signal, to the input of a stop signal caused by a triggering event, or the time interval from the input of a start signal caused by a triggering event to the subsequent end of the fixed period. Either way, there can only be one start and one stop signal within each of the measurement periods. Once an event is latched, no further events can be recorded by the TDC until it is reset at the beginning of the next period. Therefore the TDC latches the first event within each period, and subsequent events occurring in the same period are ignored. When the frequency of events is high and there is a large probability that more than one event occurs within a measurement period, the number of recorded events decreases in relation to the time elapsed since the beginning of the period in which the event occurs.
Therefore, the record of events in the time-domain shows a distinct bias, even if the events are uncorrelated like ambient noise evenly spread in time. A histogram of the number of events as a function of the time elapsed since the latest reset of the TDC will have a generally sloping shape. If only one single-photon avalanche diode (SPAD) generating the triggering events is connected to a single TDC, the relatively long deadtime of the SPAD may essentially increase the possibility that an event is missed. If a plurality of SPADs are connected to a TDC in order to extend the sensing area when the frequency of events is low, an increase of that frequency will drastically increase the probability that one of the SPADs triggers the TDC and many subsequent events generated by the other SPADs will be missed.